There are a number of solutions proposed in the prior art for calculating and regulating Optical Signal to Noise Ratio (OSNR) in optical communication networks. In particular, there are a number of approaches for controlling and regulating OSNR in optical lines comprising Raman optical amplifiers.
FIG. 1 (prior art) shows a single span exemplary optical communication system 10. In practice, such systems may be composed of several spans.
Let the illustrated system 10 initially comprises a multiplexer assembly 12 at the transmitting end of the system, a demultiplexer assembly 14 at the receiving end of the system, an optical fiber link 15 extending between an optical amplifier (EDFA) 13 the transmitting end and EDFA 17 of the receiving end for conveying the multiplexed optical signal, and a backward Raman amplifier (BRA) 18 which is inserted close to the receiving end of the link. The goal of the system designer is that OSNR at the input of any single channel receiver at the receiving end of the link is larger than the receiver OSNR Tolerance, being the minimum OSNR for which the Bit Error Rate (BER) is still better than, say, a commonly accepted standard value 10−12. The required OSNR must be greater than the receiver OSNR tolerance+a margin selected by the system designer. One of the possibilities to improve the system OSNR is to increase the input power applied to the fiber up to the possible maximum, by using output power of the EDFA's 13 booster. The possible maximum is usually set by the system nonlinearity limit.
Those skilled in the art know that, at high power levels, nonlinear phenomena like self-phase modulation (SPM), Cross-phase modulation (XPM) and four-wave mixing (FWM) cause signal distortion and performance (or BER) degradation in the system.
The non-linearity limit of a system should be understood as follows. When a network works under the non-linearity limit, non-linear penalty (performance degradation due to non-linear effects in the fiber) does not exceed a value determined by a system designer (say, 1 or 2 dB). Actually, crossing of the non-linearity limit can be expressed as such a condition of the system when increasing the power applied to the fiber leads to increase of a real OSNR required for the same stated BER. Usually, when performance of an optical line cannot be further improved by its own resources and without crossing the non-linearity limit, network designers insert a Forward Raman Amplifier (FRA) in the line.
FIG. 1 therefore illustrates inserting a FRA 16 (shown as a dashed arrow) at the beginning of the link 15.
A number of articles, for example 1) Essiambre et al. IEEE photon. Techn. Lett. Vol. 14, pp. 914 (2002); 2) Perline and Winful. IEEE photon. Techn. Lett. Vol. 14, pp. 1199 (2002) explain that the Forward Raman amplification (FRA) enables the system designer to increase the effective input power applied to the fiber, thus improving the system OSNR, without crossing the nonlinear limit.
The above articles propose various but quite complex mathematical equations which enable theoretically calculating the FRA power required for a specific communication system. However, these and some other previous works dealt with too general systems; it is very hard to employ their methods of calculation, including many system parameters, to practical systems.
To the best of the Applicant's knowledge, prior art does not give a simple and effective advice of how to estimate the required gain of FRA for obtaining a designed value of OSNR in real optical telecommunication systems. Likewise, no recommendations are found for effective OSNR regulation in real systems.